[superposeur]

At top it should read “L = …” (as it does in the compact form printed on the coffee mug image).

Here, L represents Lagrangian density. The integral of L over spacetime (between some initial and some final time) is the action S, which therefore is a functional of the particular configuration the various fields take. The classical configuration of the fields (solutions of equations of motion) is the one that minimizes S between initial and final times. Quantum mechanically, the amplitude to find final field configuration given initial field configuration is obtained by integrating e^iS over all field configurations. Modulo a few details* the workings of the universe ensue.

*joke